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<div class="section" id="forward-iterator">
<h1><a class="toc-backref" href="./iterators-concepts.html#id1450">Forward Iterator</a></h1>
<div class="section" id="id384">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>A <a class="reference internal" href="./forward-iterator.html">Forward Iterator</a> <tt class="literal"><span class="pre">i</span></tt> is a type that represents a positional reference
to an element of a <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>. It allows to access the element through
a dereference operation, and provides a way to obtain an iterator to
the next element in a sequence.</p>
<!-- A [Forward Iterator] guarantees a linear traversal over
the sequence. -->
</div>
<div class="section" id="id385">
<h3><a class="subsection-title" href="#definitions" name="definitions">Definitions</a></h3>
<ul class="simple">
<li>An iterator can be <cite>dereferenceable</cite>, meaning that <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a>&lt;i&gt;::type</span></tt>
is a well-defined expression.</li>
<li>An iterator is <cite>past-the-end</cite> if it points beyond the last element of a
sequence; past-the-end iterators are non-dereferenceable.</li>
<li>An iterator <tt class="literal"><span class="pre">i</span></tt> is <cite>incrementable</cite> if there is a &quot;next&quot; iterator, that
is, if <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a>&lt;i&gt;::type</span></tt> expression is well-defined; past-the-end iterators are
not incrementable.</li>
<li>Two iterators into the same sequence are <cite>equivalent</cite> if they have the same
type.</li>
<li>An iterator <tt class="literal"><span class="pre">j</span></tt> is <cite>reachable</cite> from an iterator <tt class="literal"><span class="pre">i</span></tt> if , after recursive
application of <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a></span></tt> metafunction to <tt class="literal"><span class="pre">i</span></tt> a finite number of times, <tt class="literal"><span class="pre">i</span></tt>
is equivalent to <tt class="literal"><span class="pre">j</span></tt>.</li>
<li>The notation [<tt class="literal"><span class="pre">i</span></tt>,<tt class="literal"><span class="pre">j</span></tt>) refers to a <cite>range</cite> of iterators beginning with
<tt class="literal"><span class="pre">i</span></tt> and up to but not including <tt class="literal"><span class="pre">j</span></tt>.</li>
<li>The range [<tt class="literal"><span class="pre">i</span></tt>,<tt class="literal"><span class="pre">j</span></tt>) is a <cite>valid range</cite> if <tt class="literal"><span class="pre">j</span></tt> is reachable from <tt class="literal"><span class="pre">i</span></tt>.</li>
</ul>
</div>
<div class="section" id="id386">
<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
<table border="1" class="docutils table">
<colgroup>
<col width="25%" />
<col width="46%" />
<col width="29%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Expression</th>
<th class="head">Type</th>
<th class="head">Complexity</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a>&lt;i&gt;::type</span></tt></td>
<td>Any type</td>
<td>Amortized constant time</td>
</tr>
<tr><td><tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a>&lt;i&gt;::type</span></tt></td>
<td><a class="reference internal" href="./forward-iterator.html">Forward Iterator</a></td>
<td>Amortized constant time</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i::category</span></tt></td>
<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a>, convertible
to <tt class="literal"><span class="pre">forward_iterator_tag</span></tt></td>
<td>Constant time</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id387">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<pre class="literal-block">
typedef <a href="./deref.html" class="identifier">deref</a>&lt;i&gt;::type j;
</pre>
<table class="docutils field-list" frame="void" rules="none">
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<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">i</span></tt> is dereferenceable</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is identical to the type of the pointed element</td>
</tr>
</tbody>
</table>
<!-- .......................................................................... -->
<pre class="literal-block">
typedef <a href="./next.html" class="identifier">next</a>&lt;i&gt;::type j;
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><tt class="literal"><span class="pre">i</span></tt> is incrementable</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is the next iterator in a sequence</td>
</tr>
<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><tt class="literal"><span class="pre">j</span></tt> is dereferenceable or past-the-end</td>
</tr>
</tbody>
</table>
<!-- .......................................................................... -->
<pre class="literal-block">
typedef i::category c;
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><tt class="literal"><span class="pre">c</span></tt> is identical to the iterator's category tag</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id388">
<h3><a class="subsection-title" href="#invariants" name="invariants">Invariants</a></h3>
<p>For any forward iterators <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> the following invariants always hold:</p>
<ul class="simple">
<li><tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent if and only if they are pointing to the same
element.</li>
<li>If <tt class="literal"><span class="pre">i</span></tt> is dereferenceable, and <tt class="literal"><span class="pre">j</span></tt> is equivalent to <tt class="literal"><span class="pre">i</span></tt>, then <tt class="literal"><span class="pre">j</span></tt> is
dereferenceable as well.</li>
<li>If <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent and dereferenceable, then <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a>&lt;i&gt;::type</span></tt>
and <tt class="literal"><span class="pre"><a href="./deref.html" class="identifier">deref</a>&lt;j&gt;::type</span></tt> are identical.</li>
<li>If <tt class="literal"><span class="pre">i</span></tt> is incrementable, and <tt class="literal"><span class="pre">j</span></tt> is equivalent to <tt class="literal"><span class="pre">i</span></tt>, then <tt class="literal"><span class="pre">j</span></tt> is
incrementable as well.</li>
<li>If <tt class="literal"><span class="pre">i</span></tt> and <tt class="literal"><span class="pre">j</span></tt> are equivalent and incrementable, then <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a>&lt;i&gt;::type</span></tt>
and <tt class="literal"><span class="pre"><a href="./next.html" class="identifier">next</a>&lt;j&gt;::type</span></tt> are equivalent.</li>
</ul>
</div>
<div class="section" id="id389">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./iterators.html">Iterators</a>, <a class="reference internal" href="./bidirectional-iterator.html">Bidirectional Iterator</a>, <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./deref.html">deref</a>, <a class="reference internal" href="./next.html">next</a></p>
<!-- Iterators/Concepts//Bidirectional Iterator |20 -->
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